[simulcast]

Summary

• render mermaid fences as diagrams by default with Diagram and Source controls
• highlight fenced code with Shiki using GitHub light and dark themes
• keep generated SVG presentation-only while preserving Mermaid as a normal editable TipTap codeBlock
• preserve multiline code whitespace when serializing CriticMarkup comments and every suggestion form
• add focused unit, component, and Playwright coverage plus README and screenshot-guide updates

Why

Roughdraft currently shows Mermaid as plain source, and its fenced-code CriticMarkup serializer routes marked code through Turndown. That path normalizes multiline whitespace and can corrupt indentation and blank lines when a code fence contains a comment or suggestion.

This keeps the Markdown source as the document of record. Mermaid rendering is lazy, uses unique render IDs and stale-result protection, runs with securityLevel: "strict", follows the system color scheme, and falls back to editable source with an actionable error. Keeping Mermaid as a normal code block is safer for source editing, comment navigation, and exact Markdown round-tripping than replacing it with an atomic diagram node.

This also acknowledges the earlier conflicted work in #102. The implementation here preserves the existing code-block schema and review behavior, and never saves generated SVG.

Validation

• pnpm --filter @roughdraft/app exec vitest run test/critic-markup.test.ts test/code-block-view.test.tsx test/render-mermaid.test.ts — 70 passed
• pnpm test:e2e packages/app/e2e/mermaid-code-blocks.spec.ts — 3 passed
• pnpm check — lint, selector policy, 387 tests, and production builds passed
• pnpm test:smoke — 12 passed
• production output keeps Mermaid core, diagram types, Shiki themes, and language grammars in separate lazy chunks
• locally reviewed through roughdraft-dev-roughdraft; the published global CLI was not changed

Fixes #115
Fixes #125

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